COLLECTIONVARRAY/NEST TABLE/INDEX BY TABLE

COLLECTIONS官方参考文档:
(http://docs.oracle.com/cd/A87860_01/doc/appdev.817/a77069/04_colls.htm#19834)
(http://www.java2s.com/Tutorial/Oracle/0520__Collections/0020__Introduction.htm)
There are three types of collections:
Varrays,Nested tables,Associative arrays (formerly known as index-by tables)
    A varray is similar to an array in Java.
You can use a varray to store an ordered set of elements having an index associated with it.
The elements in a varray are of the same type.
    A varray has one dimension.
    A varray has a maximum size that you set when creating it.
Elements in a varray can only be modified as a whole, not individually.
You can change the size of a varray later.
The elements stored in a varray are stored with the table when the size of the varray is 4KB or less, otherwise the varray is stored outside of the table.
When a varray is stored with the table, accessing its elements is faster than accessing elements in a nested table.
     A nested table is a table that is embedded within another table.
You can insert, update, and delete individual elements in a nested table.
Because you can modify individual elements in a nested table, this makes them more flexible than a varray.
    A nested table doesn't have a maximum size, and you can store an arbitrary number of elements in a nested table.
The elements for nested tables are stored in separate tables.
Associative arrays is formerly known as index-by tables.
An associative array is a set of key and value pairs.
You can get the value from the array using the key (which may be a string) or an integer.
An associative array is similar to a hash table.
You create a collection type using the SQL DDL CREATE TYPE statement.
Then you can use these types to define columns in a table.
    An associative array is a PL/SQL construct, not a SQL construct.
    An associative array cannot be stored persistently in a table.
You might be asking yourself why you would want to use collections in the first place.
After all, using two tables with a foreign key already allows you to model relationships between data.
The answer is that the data stored in the collection may be accessed more rapidly by the database than if you were to use two tables instead.
Typically, you'll want to use a collection if you have data that is only used by one table.
A collection type whose elements are also a collection type is known as a multilevel collection type.
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The following list shows the valid multilevel collection types:
A nested table containing a nested table type
A nested table containing a varray type
A varray containing a varray type
A varray containing a nested table type
A varray or nested table of an object type that has an attribute that is a varray or nested table type
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A series of collection methods can be used to determine the size, and the rows populated, in any collection datatype: index-by tables, VARRAYs, and nested tables. The following is a list of the collection methods and their purposes:

EXISTS(row) returns TRUE if the row specified exists.
COUNT returns the number of rows.
FIRST returns the row number of the first populated row.
LAST returns the row number of the last populated row.
PRIOR(row) returns the row number of the last row populated before the row specified.
NEXT(row) returns the row number of the next row populated after the row specified.
DELETE removes all rows.
DELETE(row) removes the specified row.
DELETE(start_row,end_row) removes all rows between and including the start_row and end_row.
TRIM removes the last row.
TRIM(n) removes the last n rows.
EXTEND adds one row.
EXTEND(n) adds n rows.
EXTEND(n,m) adds n copies of row m.

                                                         三种COLLECTION的比较
                                              http://docstore.mik.ua/orelly/oracle/prog2/ch19_01.htm

Table 19.2: Comparing Oracle Collection Types

Characteristic	Index-By Table	Nested Table	VARRAY
Dimensionality	Single	Single	Single
Usable in SQL?	No	Yes	Yes
Usable as column datatype in a table?	No	Yes; data stored "out of line" (in separate table)	Yes; data stored "in line" (in same table)
Uninitialized state	Empty (cannot be null); elements undefined	Atomically null; illegal to reference elements	Atomically null; illegal to reference elements
Initialization	Automatic, when declared	Via constructor, fetch, assignment	Via constructor, fetch, assignment
In PL/SQL, elements referenced via	BINARY_INTEGER (-2,147,483,647 .. 2,147,483,647)	Positive integer between 1 and 2,147,483,647	Positive integer between 1 and 2,147,483,647
Sparse?	Yes	Initially, no; after deletions, yes	No
Bounded?	No	Can be extended	Yes
Can assign value to any element at any time?	Yes	No; may need to EXTEND first	No; may need to EXTEND first, and cannot EXTEND past upper bound
Means of extending	Assign value to element with a new subscript	Use built-in EXTEND procedure (or TRIM to condense), with no predefined maximum	EXTEND (or TRIM), but only up to declared maximum size
Can be compared for equality?	No	No	No
Retains ordering and subscripts when stored in and retrieved from database?	N/A	No	Yes

The inevitable question is: Which construct should I use? This chapter reviews some examples of the new collections and offers some suggestions in this area. The short answer:

• Nested tables are more flexible than VARRAYs for table columns.

• VARRAYs are best when you need bounded arrays that preserve element order.

• Index-by tables are the only option that allows initial sparseness.

• If your code must run in both Oracle7 and Oracle8, you can use only index-by tables.